Solution and method for producing the same, and a method for producing active material for secondary battery

ABSTRACT

There is provided a solution containing lithium and at least one of a niobium complex and a titanium complex, excellent in storage stability, and suitable for forming a coating layer capable of improving battery characteristics of an active material, and a related technique, which is the solution containing lithium, at least one of a niobium complex and a titanium complex, and ammonia, wherein an amount of the ammonia in the solution is 0.2 mass % or less.

TECHNICAL FIELD

The present invention relates to a solution exhibiting suitable storagestability and battery characteristics for forming a positive electrodeactive material of a secondary battery, a method for producing the same,and a method for producing the positive electrode active material forforming the secondary battery formed using the above solution.

DESCRIPTION OF THE RELATED ART

Lithium ion battery is characterized by high energy density and capableof being operated at high voltage. Therefore, it is used as aninformation device such as a mobile phone as a secondary battery whichis easy to be reduced in size and weight. Further, in recent years,demands for secondary batteries for large-scale power such as for hybridvehicles are also increasing.

In the lithium ion battery, a nonaqueous solvent electrolyte in which asalt is dissolved in an organic solvent is generally used as anelectrolyte. However, since the nonaqueous solvent electrolyte isflammable, there is a necessity for solving a problem on safety in usingthe lithium ion battery. In order to ensure such safety, for example,measures such as incorporating a safety device into the lithium ionbattery are being implemented. Further, as a more fundamental solution,there has been proposed a method for using the abovementionedelectrolyte as a nonflammable electrolyte, that is, a method for forminga lithium ion conductive solid electrolyte.

Generally, an electrode reaction of a battery occurs at the interfacebetween the electrode active material and the electrolyte. Here, when aliquid electrolyte is used for the electrolyte, by immersing theelectrode containing the electrode active material in the liquidelectrolyte, the liquid electrolyte penetrates between active materialparticles to form a reaction interface. In contrast, when a solidelectrolyte is used for the electrolyte, the solid electrolyte has nopenetration mechanism between such active material particles, andtherefore it is necessary to mix a powder containing the electrodeactive material particles and a powder of the solid electrolyte inadvance. Therefore, the positive electrode of an all-solid-state lithiumion battery is usually a mixture of the positive electrode activematerial powder and the solid electrolyte.

However, in the all-solid-state lithium ion battery, resistancegenerated when lithium ions migrate at an interface between the positiveelectrode active material and the solid electrolyte (sometimes referredto as “an interface resistance” hereafter), is likely to be increased.When the interface resistance is increased, a performance such as abattery capacity is deteriorated in the all-solid-state lithium ionbattery.

Here, non-patent document 1 discloses that the increase of the interfaceresistance is caused by a reaction of the positive electrode activematerial and the solid electrolyte to form a high resistance portion onthe surface of the positive electrode active material. Non-patentdocument 1 also discloses that the interface resistance is reduced bycoating the surface of lithium cobalt oxide which is the positiveelectrode active material, with lithium niobate, to thereby improve theperformance of the all-solid-state lithium ion battery.

Non-patent document 2 discloses that the interface resistance is reducedby coating the surface of lithium cobalt oxide with lithium titanate, tothereby improve the performance of the all-solid-state lithium ionbattery.

Specifically, non-patent document 2 discloses that an alcohol solutionmixed with metal alkoxide such as Nb alkoxide, Ti alkoxide, Li alkoxideor the like is brought into contact with a lithium-metal oxide surfacesuch as lithium cobalt oxide, and thereafter the lithium-metal oxide isbaked in the atmosphere, to thereby coat the surface with lithiumniobate or lithium titanate.

In contrast, patent document 1 also discloses a method for producinglithium cobaltate coated with lithium niobate. Specifically, an alcoholsolution mixed with a metal alkoxide such as Nb ethoxide or Li ethoxideis brought into contact with the surface of lithium cobaltate, and thislithium cobaltate was baked at a relatively low temperature of 260° C.to 300° C. by low-temperature baking, wherein the interfacial resistanceof a coating layer is reduced by suppressing a crystallization oflithium cobaltate coated with lithium niobate.

Patent document 2 also discloses a method for producing lithiumcobaltate coated with lithium niobate using a solution containinglithium and a niobium complex. The present inventors further disclose inpatent documents 3 and 4, a solution containing lithium and a niobiumcomplex which is less likely to form a precipitate and is excellent instorage stability, and a method for producing the same.

PRIOR ART DOCUMENT Patent Document

Patent Document 1

-   [Patent Document 1] Japanese Patent Application Laid-Open No.    2010-129190-   [Patent Document 2] Japanese Patent Application Laid-Open No.    2012-074240-   [Patent Document 3] Japanese Patent Application Laid-Open No.    2014-210701-   [Patent Document 4] Japanese Patent Application Laid-Open No.    2015-103321

Non-Patent Document

-   [Non-Patent Document 1] Electrochemistry Communications, 9 (2007) p.    1486 to 1490-   [Non-Patent Document 2] Advanced Materials, 18 (2006) p. 2226 to    2229

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

As described above, in Patent Documents 3 and 4, the inventors of thepresent invention disclose a solution containing lithium and a niobiumcomplex which is less likely to form a precipitate and is excellent instorage stability. According to this technique, in a method for forminga lithium niobate compound on the surface of the positive electrodeactive material by using a niobium complex, a dramatic improvement isachieved in the storage stability of the solution containing the niobiumcomplex which is regarded as an urgent subject.

In the step of forming the niobium complex, ammonia is an essentialconstituent. Conventionally, it is conceivable that if the amount ofammonia in the solution becomes too small, the niobium complex which issupposed to be originally necessary is changed to insoluble niobiumhydroxide, and therefore ammonia is preferably remained in the liquid.

However, it has become clear that excessive ammonia in the solutioncauses a chemical damage to the active material, which causesdeterioration of battery characteristics as a result.

Further, in order to obtain a stable complex in the course of itsformation, it is necessary to charge 2 mol or more of ammonia withrespect to 1 mol of niobium atoms, but much ammonia will be inevitablyremained in the solution as a result. According to a conventionalproduction method, there is no removal step of removing excessiveammonia, and therefore much ammonia is remained in the solution aftercomplex synthesis. Due to a great contribution to a stabilization of theniobium complex, ammonia is not specifically removed originally.

Therefore, in order to solve the abovementioned problem, inventors ofthe present invention study on a technique of providing a solutioncontaining lithium and at least one of a niobium complex and a titaniumcomplex, that is, a solution which is excellent in storage stability andwhich is suitable for forming a coating layer capable of improving thebattery characteristics of the active material.

Means for Solving the Problem

According to the study by the present inventors in order to solve theabove problem, there are inventions as follows.

A first invention of the present invention provides a solutioncontaining lithium, at least one of a niobium complex and a titaniumcomplex, and ammonia, wherein an amount of the ammonia in the solutionis 0.2 mass % or less.

A second invention of the present invention provides the solution of thefirst invention, wherein the metal complex has a peroxy group.

A third invention of the present invention provides the solution of thesecond invention, wherein a molar ratio of atoms in the lithium to atomsof a metal in the metal complex is 0.8 to 2.0.

A fourth invention of the present invention provides the solution of anyone of the first to third inventions, which further contains a reducingcompound.

A fifth invention of the present invention provides the solution of thefourth invention, wherein a ratio of the reducing compound in thesolution is 0.01 mass % to 5.0 mass %.

A sixth invention of the present invention provides the solution of anyone of the first to fifth inventions, wherein an amount of the ammoniain the solution is 0.1 mass % or less.

A seventh invention of the present invention provides a method forproducing an active material for a secondary battery, including:

performing a surface treatment using the solution of any one of thefirst to sixth inventions; and

heat-treating the surface-treated active material.

An eighth invention of the present invention provides the method of theseventh invention, wherein the active material is an oxide containinglithium.

A ninth invention of the present invention provides the method of theseventh or the eight invention, wherein the active material is an oxidecontaining lithium, and at least one of a lithium niobate compound and alithium titanate compound is attached to a main surface of the activematerial.

A tenth invention of the present invention provides a method forproducing a solution, including:

forming a metal complex in a solution by mixing at least one of niobicacid and titanic acid with ammonia;

mixing the metal complex and a lithium compound in the solution; and

removing the ammonia in the mixed solution until an amount of theammonia is reduced to 0.2 mass % or less.

By using an active material having a coating layer formed by using asolution of the present invention, it is possible to obtain a secondarybattery having excellent battery characteristics. Further, it ispossible to obtain at least one of a niobium complex solution and atitanium complex solution for surface treating (coating) an activematerial for a secondary battery which is excellent in storagestability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural formula of citric acid.

FIG. 2 is a structural formula of EDTMPA.

FIG. 3 (a) is a photograph showing a result of SEM observation of across section of a particle constituting the active material, FIG. 3 (b)is a view showing a result of Co (cobalt) element mapping for the activematerial, and FIG. 3 (c) is a view showing a result of Nb (niobium)element mapping.

DETAILED DESCRIPTION OF THE INVENTION

As a mode for carrying out the present invention, explanation will begiven for a solution containing lithium and a niobium and/or titaniumcomplex, a form of a complex in the solution, a component to be added tothe solution, a post-treatment method performed to the solution,lithium-metal oxides such as lithium cobaltate coated with lithiumniobate and/or lithium titanate using the obtained solution, and amethod for producing the same, respectively.

“A and/or B” means at least any one of A and B hereafter. Particularly,the niobium complex and/or the titanium complex are sometimes simplyreferred to as “a metal complex”. Further, “to” means that it is notless than a predetermined numerical value and not more than apredetermined numerical value in this specification.

(A Solution Containing Lithium and the Niobium Complex and/or theTitanium Complex)

The solution containing lithium and the niobium complex and/or thetitanium complex (metal complex) of the present invention, can beobtained by mixing a solution containing a water-soluble metal complexwith a lithium compound such as lithium salt.

(Niobium Complex and/or Titanium Complex)

A ligand of the niobium complex and/or the titanium complex may be anyone as long as the complex becomes soluble in water, and further, when asurface coat layer is formed, it is preferable to select the ligandwhich does not generate residual carbon that causes deterioration ofbattery characteristics. When a material containing carbon in the metalcomplex is selected, it is preferable to select the material having aproperty that the metal complex is desorbed from the active material, inthe step of performing surface treatment to the active material for asecondary battery described later using the solution of the presentinvention to form a coating layer on a main surface of the activematerial, and thereafter heat-treating (baking) the active material inthe atmosphere. More specifically, it is preferable to select thematerial having a property that the complex is decomposed during baking.Accordingly, although sometimes influenced by baking conditions, it ispreferable to select niobium complex and/or titanium complex having aproperty of being decomposed at 750° C. or less, preferably 650° C. orless, more preferably 300° C. or less.

The abovementioned niobium complex and/or titanium complex preferablyhas a peroxy group. The peroxo complex does not contain carbon in itschemical structure, and therefore when the surface coating layer of thepositive electrode active material is formed via such a complex, it isnot influenced by a baking temperature, and a carbon residue such ascausing deterioration of battery characteristics is not generated, whichis preferable.

The peroxo complex of niobium and/or titanium can be prepared, forexample, by the following method. Patent Document 4 may be referred tofor contents not included below. The term peroxo complex as referred toherein refers to one having a peroxy group (—O—O—) bonded to niobiumand/or titanium. However, all of the ones bonded to niobium and/ortitanium are not required to be peroxy groups, and for example most ofthose bonded to niobium and/or titanium may be peroxy groups, while somemay remain oxygen (oxy group).

The peroxo complex of niobium is obtained by adding ammonia water toniobic acid (diniobium pentoxide hydrate) and further adding hydrogenperoxide. When the peroxo complex of niobium is prepared, an excessiveamount of ammonia water is added, for example, at a molar ratio of 2 molor more, preferably 3 mol or more, with respect to 1 mol of the niobiumatom.

When the peroxo complex of niobium is prepared, it is preferable that anexcessive amount of hydrogen peroxide is added to niobic acid.Specifically, hydrogen peroxide is at least 10 moles, preferably atleast 30 moles, more preferably at least 50 moles, per 1 mole of niobiumatom in molar ratio. With this molar ratio, it is possible to inhibithydrolysis of the peroxo complex from predominating, although the peroxocomplex is supposed to be prepared, and a desired niobium peroxo complexcan be surely obtained, which is preferable.

In contrast, when the peroxo complex of titanium is prepared, the peroxocomplex is obtained by adding ammonia water to metal titanium (which maybe powder or foil) and further adding hydrogen peroxide. When the peroxocomplex of titanium is prepared, an excessive amount of ammonia water isadded, for example, at a molar ratio of 2 moles or more, preferably 3moles or more, per 1 mole of the titanium atom. In the same manner asniobic acid, it is preferable to add an excessive amount of hydrogenperoxide. Specifically, the ratio of hydrogen peroxide is 10 moles ormore, preferably 30 moles or more, more preferably 50 moles or more ofhydrogen peroxide per 1 mole of titanium.

By the above method, the peroxo complex of niobium and/or titanium canbe obtained. The solution containing the peroxo complex is transparent.

(Lithium Compound)

By adding a lithium compound to an aqueous solution containing the metalcomplex obtained by the abovementioned method, a solution containinglithium and the metal complex can be completed. The number of moles oflithium of the lithium compound to be added can be arbitrarily set withrespect to the number of moles of niobium and/or titanium in the metalcomplex contained in the aqueous solution.

However, preferably an amount of lithium atoms is preferably in a rangeof 0.8 to 2.0 moles, per 1 mol of niobium and/or titanium atoms.Further, an amount of lithium atoms may be set in a range of 0.8 to 1.5moles with respect to 1 mole in total of niobium and/or titanium atoms.Further, an amount of lithium atoms may be set in a range of 0.8 to 1.2moles, with respect to 1 mole in total of niobium atoms. Moreover, anamount of lithium atoms may be in a range of 0.8 to 1.5 moles, withrespect to 1 mole in total of titanium atoms.

When the amount of lithium is a lower limit value or more with respectto the amount of niobium and/or titanium, lithium conductivity oflithium niobate and/or lithium titanate obtained from the metal complexcan be maintained at an appropriate value. When the amount of lithium isan upper limit value or less with respect to the amount of niobiumand/or titanium, this is appropriate because it is unnecessary to haveexcessive lithium not involved in lithium conductivity.

As a preferable example of the lithium compound to be added, inorganiclithium salts such as lithium hydroxide (LiOH), lithium nitrate (LiNO₃),lithium sulfate (Li₂SO₄) and lithium carbonate (Li₂CO₃), etc., can beused.

(Other Additives)

In order to further improve the stability of the metal complex, it ispreferable to add a stability improver which is a reducing compound, tothe solution of the present invention (see, for example, the stabilityimprover in Patent Document 4). A structure in which carboxylic acids,dicarboxylic acids, hydroxycarboxylic acids, and phosphonic acids areadded as the stability improver is also a preferable form. It isconceivable that the carboxylic acid has a —COOH group and is bonded tothe niobium complex at one site. Preferable examples of the carboxylicacid include formic acid and acetic acid.

The dicarboxylic acid has two —COOH groups, and the hydroxycarboxylicacid has —OH group and —COOH group. Then, it is conceivable that thesegroups are bonded to the niobium complex and/or the titanium complex atone or two or more sites. Oxalic acid ((COOH)₂) is used as thedicarboxylic acid, and citric acid (C₆H₈O₇, structural formula is shownin FIG. 1) which is a hydroxy tricarboxylic acid and malic acid (HOOC—CH(OH)—CH₂—COOH) which is a hydroxydicarboxylic acid are used as thehydroxycarboxylic acid, as preferable examples.

Similarly, compounds such as phosphonic acids having two or more groupscapable of bonding to the metal complex (particularly niobium complex)are effective. The phosphonic acids can be bonded to the niobium complexat one or two or more sites depending on the number of groups capable ofbonding to the niobium complex. EDTA ((HOOCCH₂)₂NCH₂CH₂N(CH₂COOH)₂) andEDTMPA (Ethylene Diamine Tetra (Methylene Phosphonic Acid), which isshown in FIG. 2), are used as preferable examples of the phosphonicacid.

As the stability improver which can be added to the solution of thepresent invention, the group bonded to the abovementioned metal complexincludes a carboxyl group, an alcoholic hydroxyl group, a phosphinogroup, and an amino group, etc. In the stability improver of the presentinvention, O (oxygen), N (nitrogen), and P (phosphorus) are bonded tothe metal complex. Then, it is conceivable that the stability improverof the present invention is positioned so as to surround the metalcomplex to stabilize the metal complex.

Further, when the stability improver which can be added to the solutionof the present invention is a chelate compound having these groups inthe molecular structure in a complex manner, it is conceivable that itcoordinates with niobium and/or titanium in the metal complex, and aneffect of improving stability can be expected, which is preferable.

Then, the present invention has a great characteristic in a point thatin the solution containing lithium and the abovementioned metal complex,the residual amount of ammonia is 0.2 mass % or less while ammonia isremained. The residual amount of ammonia is inversely correlated withthe battery characteristics, and the higher the concentration is, theworse the battery characteristics are. Incidentally, batterycharacteristics of the active material can be measured either in an allsolid state battery or a Li ion battery, as shown in the items of theexamples below. Any type of electrolyte or negative electrode isacceptable as long as it can measure the battery characteristics of theactive material. Conveniently, Li foil is used for the negativeelectrode, and LiPF₆ dissolved in an organic solvent is used as anelectrolyte, and the battery characteristics can be measured bypreparing a half cell. As the battery characteristics to be handled, avalue (referred to as a change rate) obtained by dividing a dischargecapacity B at the time of discharging at a high rate (3 C) by adischarge capacity A at the time of discharging at a low rate (0.1 C)may be used. The larger the value is, the more smoothly the exchange ofthe lithium ion of the active material is carried out, which means thatresistance of the battery is low.

Here, in view of a relationship between the residual amount of ammoniaand the battery characteristics, the residual amount of ammonia is setto 0.2 mass % or less (preferably 0.1 mass % or less, more preferably0.05 mass % or less) in the present invention. Thus, in a case of acoating applied to the active material a secondary battery excellent inbattery characteristics such as a good change rate in discharge capacityand a low DC resistance value can be obtained, for example as shown inexamples described later.

Further, when the metal complex is the niobium peroxo complex and/or thetitanium peroxo complex, hydrogen peroxide will be added to prepare theperoxo complex. However, decomposition of hydrogen peroxide is sometimespromoted by ammonia in a storage environment at a high temperatureexceeding 40° C. Therefore, by reducing the amount of ammonia in thesolution containing the metal complex, decomposition of hydrogenperoxide by ammonia can be reduced, and a stable solution can beobtained even at a high temperature.

Ammonia is decomposed by, for example, an ion exchange method, reducedpressure, heating, or catalyst (nickel or platinum group catalyst) sothat the residual amount of ammonia becomes 0.2 mass % or less, and theresidual amount of ammonia can be appropriately changed. Particularly,when ammonia is removed by ion exchange, the above methods arepreferable because they are relatively inexpensive methods. Thesemethods are not limited to one kind, and may be performed incombination.

The ion exchange method includes a method using an ion exchange resinand a method using zeolite, and either one of them may be selected.However, it is preferable to select an adsorbent excellent in ammoniaselectivity. As zeolite excellent in ammonia selectivity, clinoptilolite(Ca, Na₂) [Al₂Si₇O₁₈].16H₂O or mordenite (Ca, K₂, Na₂) [AlSi₅O₁₂]₂.7H₂O,etc., are known. Among them, especially clinoptilolite has excellentammonia adsorption performance.

However, when the amount of ammonia is too small, the abovementionedmetal complex becomes unstable. This is attributed to the fact that theperoxy group of the metal complex is decomposed by hydrolysis and islikely to release hydroxyl ions. On the other hand, when theconcentration of ammonia in the solution containing the metal complex ishigh to some extent, it is conceivable that the metal complex can bestabilized because the hydroxyl ion concentration can be made moderatelyhigh.

As described above, the above metal complex is extremely unstable in atransient state during preparation of a complex, and it is necessary tosuppress a hydrolysis reaction of the complex in the presence ofexcessive ammonia. However, the excessive ammonia as much as the amountof the ammonia at the time of preparing the metal complex, is notrequired for the metal complex to which lithium is added. However,ammonia is required to be remained to such an extent that stability oflithium-containing niobium and/or titanium complex can be secured.

The residual amount (mass %) of ammonia in the solution at that time ispreferably 10 ppb or more, more preferably 1 ppm or more, still morepreferably 10 ppm or more. Of course, the abovementioned condition of0.2 mass % or less is required to be satisfied. With the residual amount(concentration) in this range, there is almost no decomposition ofhydrogen peroxide by ammonia, which contributes to the stability(storage stability) of the peroxo complex of niobium and/or titaniumcontaining lithium.

The amount of ammonia contained in the solution can be obtained, forexample, by ion chromatography or absorbance method, a titration method,or the like.

Then, in the present invention, in addition to the abovementioned liquidcomposition, the residual amount of hydrogen peroxide is preferably 1mass % or less. The self-decompositin rate of hydrogen peroxide isproportional to the concentration of hydrogen peroxide in the liquid,and the higher the concentration is, the faster the decomposition rateis. Based on such a knowledge, the residual amount of hydrogen peroxidein the solution is set to 1 mass % or less in the present invention. Inthis manner, the self-decomposition rate of hydrogen peroxide ispractically negligible when the solution is stored, and consequently itbecomes possible to suppress change of the composition of the solutionand the corrosion of the equipment due to self-decomposition. Further,as a secondary effect, it becomes possible to use the abovementionedsolution also in a case of a coating applied to an active material whichis chemically weak against hydrogen peroxide (such as lithiumnickelate).

The residual amount of hydrogen peroxide can be appropriately varied,for example so that it is 1 mass % or less in the solution, bydecomposing hydrogen peroxide, for example by ultraviolet irradiation orunder reduced pressure, or by heating or enzyme (catalase).Particularly, when hydrogen peroxide is decomposed by ultravioletirradiation, it is conceivable that excessive carbon remaining in thesolution can also be decomposed because formation of hydroxyl radicalsis promoted, which is preferable. Further, the decomposition of hydrogenperoxide by ultraviolet irradiation is preferable because it has littleinfluence on other compounds. These methods are not limited to one kind,and may be performed in combination.

However, when the amount of hydrogen peroxide is too small, the metalcomplex becomes unstable. This is attributed to the fact that the peroxygroup of the metal complex is easily decomposed by hydrolysis.Specifically, when the concentration of hydrogen peroxide in thesolution containing the metal complex is high to some extent, even ifthe peroxy group is removed from the above niobium complex and/ortitanium complex by hydrolysis, the peroxy group is newly supplementedto the coordination site of niobium and/or titanium. As a result, thestability of the niobium complex and/or the titanium complex ismaintained. Conversely, when there is little hydrogen peroxide in thesolution, there is no way to supplement the decrease due to hydrolysis,and therefore the form of the complex is collapsed, and as a result,niobium hydroxide or the like is formed, and the amount of peroxocomplex of niobium in the solution becomes insufficient.

Namely, in a transient state during preparation of the complex, theperoxo complex related to the present invention becomes extremelyunstable, and it is necessary to suppress the hydrolysis reaction of thecomplex in the presence of a large amount of excessive hydrogenperoxide. However, in the metal complex to which lithium is added, thestability of the complex is improved, and therefore the excessiveammonia as much as the amount of the ammonia at the time of preparingthe metal complex, is not required. However, it is necessary to makehydrogen peroxide remained to the extent that stability of niobiumand/or titanium complex containing lithium can be secured.

According to the study of the present inventors, it is found that theabovementioned effect is exhibited by allowing 1 mass % or less,preferably 10 ppb or more, more preferably 1 ppm or more, still morepreferably 10 ppm or more of hydrogen peroxide to be remained in thesolution at that time. With the residual amount (concentration) in thisrange, there is almost no self-decomposition of hydrogen peroxide, andthe stability of the peroxo complex of niobium and/or titaniumcontaining lithium is insured.

An amount of hydrogen peroxide contained in the solution is obtained by,for example Ti-PAR absorption spectrometry, titration method usingpotassium permanganate or iodine, voltammetric method, and Post-columnHPLC method using chemiluminescence detector.

Further, the peroxy group in the peroxo complex of niobium and/ortitanium containing lithium can be confirmed by presence or absence of apeak derived from the O—O bond in the vicinity of 880 cm⁻¹ when forexample a precipitate (crystal of lithium and a niobium complex and/or atitanium complex) obtained by adding 10 g of a solution into 100 ml ofisopropanol, is measured by a Fourier transform infrared absorptionspectrum measuring apparatus or a Raman spectroscopic apparatus.

<Method for Adding Stability Improver into Solutions Containing Lithiumand Niobium Complex and/or Titanium Complex>

As a method for adding a substance which further induces the effect ofimproving stability of a solution containing lithium and niobium and/ora titanium complex, citric acid monohydrate which ishydroxytricarboxylic acid will be explained as an example. Citric acidmonohydrate (C₆H₈O₇. H₂O) may be added to the abovementioned aqueoussolution containing lithium and niobium complex, in an amount of 0.01mass % to 5.0 mass %.

Here, as the form of citric acid to be added, citric acid anhydride canbe used in addition to monohydrate. However, from the viewpoint ofsolubility in water, it is preferable to use citric acid monohydratehaving high solubility.

When the addition amount is 0.01 mass % or more, the effect of improvingthe stability can be obtained. In contrast, when the addition amount is5 mass % or less, it is possible to obtain adequate content of C(carbon) which becomes an impurity in the subsequent step while exertingthe effect of improving the stability.

As described above, explanation has been given for a mode of adding astability improver into the solution containing lithium and niobiumand/or a titanium complex, as an example of citric acid monohydratewhich is hydroxytricarboxylic acid. This mode is not particularlylimited to citric acid, and can respond to a case of using thecarboxylic acids, dicarboxylic acids, other hydroxycarboxylic acids, andphosphonic acids exemplified as described above, in the same manner as acase of using citric acid monohydrate.

(Storage Stability of the Solution Added with a Stability Improver andContaining Lithium, Niobium Complex and/or Titanium Complex)

It is confirmed by the inventors of the present invention that thesolution of the present invention added with a stability improver andcontaining lithium, niobium complex and/or titanium complex, hasexcellent storage stability in which no precipitate is formed even ifbeing left standing for 12 hours or more after production.

As a result, when surface treatment is performed to the active material(lithium-metal oxide such as lithium cobalt oxide or the like) for thesecondary battery using the above solution, and coating, namely, a stepof attaching the lithium niobate compound and/or the lithium titanatecompound to the main surface of the active material is performed, acoating amount of lithium niobate and/or lithium titanate can be insuredand control becomes easy. Then, it is also possible to avoid problemssuch as mixing of the precipitate into the active material for thesecondary battery coated with the lithium niobate and/or the lithiumtitanate. Further, there is less necessity to start the process ofcoating the active material for the secondary battery within a certainperiod of time after preparing the solution, and the productionefficiency can be improved.

Active materials such as lithium nickel oxide (LiNiO₂), lithiummanganate (LiMnO₄), (LiNi_(0.95) Al_(0.05) O₂ and the like) in which apart of transition metals of these active materials are substituted withAl, Ti, Cr, Fe, Zr, Y, W, Ta, Nb, and active materials (LiNi_(1/3)CO_(1/3) Mn_(1/3) O₂, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂,LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, LiNi_(0.5)Mn_(1.5)O₄, etc.) obtained bycombining the above active materials, can be used for the activematerial for a secondary battery, other than lithium cobalt oxide(LiCoO₂).

(Baked Lithium-Metal Oxide after its Surface is Coated with a SolutionContaining Lithium and Niobium and/or a Titanium Complex to which aStability Improver has been Added).

The active material for constituting a secondary battery is coated witha solution containing lithium and niobium complex and/or titaniumcomplex stabilized by adding citric acid monohydrate or the like, andthereafter an appropriate heat treatment (for example, baking) isapplied thereto, to thereby decompose a component containing elementssuch as C, N, S and P in the additive and remove it to such an extentthat practically no problem is caused.

As a method for coating the active material with the solution, apublicly-known method can be used, such as a method for spraying asolution on an active material, a method for immersing an activematerial in a solution to dry it, and a method for dispersing an activematerial in an organic solvent and adding the solution thereinto.

After decomposition, the surface of the active material is coated withlithium composite oxide of titanium and/or niobium. Presence or absenceof these oxides can be confirmed, for example, by cutting the particleinto a cross section and observing the segregation of titanium and/orniobium on the particle surface part using SEM-EDX.

As a result, even when the positive electrode active material forconstituting a secondary battery coated with the solution is used as thepositive electrode material of the lithium ion battery, it is possibleto avoid influencing the battery characteristics.

Accordingly, the lithium-metal oxide baked after the surface is coatedwith the solution containing lithium and niobium and/or titanium complexto which the stability improver of the present invention is added, issuitable as a positive electrode active material of the secondarybattery.

Although the metal complex having a peroxy group has been mentioned as apreferable example, the stability of the metal complex can be secured tosome extent as long as the above stability improver is added, even ifthe metal complex does not have a peroxy group. In this case, theresidual amount of ammonia can be reduced to 0.2 mass % or less whileallowing ammonia to be remained in the solution without excessivelyimpairing the stability.

As described above, embodiments of the present invention have beendescribed. However, the present invention is not limited in any way tothe abovementioned embodiment, and can be variously modified within thescope not deviating from the gist of the present invention.

EXAMPLES

Examples of the present invention and comparative examples will bedescribed hereafter.

In the following, the amounts of ammonia in the examples and comparativeexamples were measured by ion chromatography (ICS-3000 type). IonPac CS14 was used as a cation molecule column and 10 mmol/L of metasulfonicacid (both produced by Dionex Corporation) was used as an eluent.

Further, a spectrophotometric device manufactured by HitachiHigh-Technologies Corporation was used for measuring the amount ofhydrogen peroxide. The measurement was performed as follows:quantitative analysis of H₂O₂ in the test solution was performed bymeasuring intensity at a measurement wavelength of 520 nm and obtaininga relative intensity with respect to a standard solution of H₂O₂, usingTi-PAR absorption photometric method (measurement wavelength: 520 nm).

Further, regarding the presence or absence of the peroxy group in theniobium complex and/or the titanium complex of the examples andcomparative examples, the presence of the peroxy group in all theexamples and comparative examples was confirmed by confirming thepresence or absence of a peak derived from the O—O bond near 880 cm⁻¹when 10 g of the solution was added into 100 ml of isopropanol and theobtained precipitate (crystals from lithium and niobium complex and/ortitanium complex) was measured using a Fourier transform infraredabsorption spectrum analyzer (NICOLET 6700 instrument manufactured byThermo SCIENTIFIC).

In the present specification, the residual amount of ammonia is set to0.2 mass % or less. Therefore, strictly speaking, examples 1, 2, 3, and6 in which the residual amount of ammonia exceeds 0.2 mass % areexamples for reference. However, for the sake of convenience ofexplanation, sequential numbers are attached to the examples.

Comparative Example 1

A hydrogen peroxide aqueous solution was prepared, in which 20.0 g ofhydrogen peroxide water having a concentration of 30 mass % was added to33.5 g of pure water. 2.01 g of niobic acid (Nb₂O₅.5.5H₂O (72.6% contentof Nb₂O) was added to the hydrogen peroxide aqueous solution. Afteraddition of the niobic acid, the temperature of the liquid to which theniobic acid was added was adjusted so that the liquid temperature waswithin a range of 20° C. to 30° C. To this liquid to which niobic acidwas added, 3.3 g of aqueous ammonia having a concentration of 28 mass %was added and sufficiently stirred to thereby obtain a transparentsolution.

In a nitrogen gas atmosphere, 0.46 g of lithium hydroxide.monohydrate(LiOH.H₂O) was added to the obtained transparent solution, to therebyobtain a transparent aqueous solution containing lithium and a peroxocomplex of niobium. The molar ratio of atoms in lithium to the niobiumatoms in the metal complex contained in this aqueous solution is 1.0.

Thereafter, the aqueous solution containing lithium and a niobiumcomplex was allowed to stand at a temperature of 25° C. for apredetermined time (6 hours to 168 hours), and whether or not aprecipitate was formed was visually confirmed. When the precipitate wasformed, the liquid was stirred to such a degree that the precipitate wasdispersed, and thereafter filtration was performed using a membranefilter having a pore size of 0.5 μm, to thereby obtain the solutioncontaining lithium and a peroxoniobic acid complex. At this time, anamount of ammoniain the solution was 1.5 mass %.

Example 1

4 g of zeolite (Itaya zeolite Z-13 manufactured by ZEEKLITE Co.) wasadded to a transparent aqueous solution containing lithium and a peroxocomplex of niobium (before being allowed to stand at 25° C. for apredetermined time) in which lithium hydroxide monohydrate was mixed,and the mixture was stirred for 30 minutes and centrifuge filtered, tothereby obtain a solution in which ammonium ions in the solution wereremoved (ammonia was removed). The amount of ammonia was measured in thesame manner as comparative example 1. At this time, the amount ofammonia was 0.3 mass % (the moisture adsorbed on the zeolite wassupplemented with pure water, and adjusted to the same weight of thesolution before the removal treatment).

Unlike a colloidal solution (sol solution), this solution was atransparent solution in which Tyndall phenomenon due to scattered lightwas not observed. Also, even after storing this solution at 25° C. for 1month, turbidity of the solution or formation of precipitate due todecomposition of the niobium complex was not observed, and the solutionwas remained to be a clear solution.

The solution immediately after the ammonium ion was removed in this way,was sprayed for 2 hours by misting, while 100 g of nickel cobalt lithiummanganate powder (LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ produced by MTI particlesize 13 μm, BET 0.353 m²/g) as an active material for a secondarybattery was heated to 100° C., and thereafter dried in the air at 100°C. for 6 hours. Thereafter, baking was performed at 300° C. for 1 hour,to thereby obtain an active material for a secondary battery to whichsurface treatment was applied.

<Details of Chemical Analysis>

As a result of analyzing the niobium content in the active material ofexample 1 by ICP, the niobium content was 0.96 mass %. Since atheoretical value of the niobium content was 1.00 mass %, when assumingthat the niobium complex was entirely attached to the active material aslithium niobate (LiNbO₃), an attachment yield was calculated to be 96%,and it was confirmed that almost all the niobic acid complex was adheredto the active material surface.

FIG. 3 shows results of various observations performed to the activematerial of example 1 in which the niobic acid complex was adhered tothe surface of the active material. FIG. 3 (a) is a photograph showingthe result of SEM observation (SEM-EDX, apparatus JSM-7800 Fmanufactured by JEOL Ltd.) performed to a cross section of a particleconstituting the active material, and it was confirmed that coating wasperformed to the active material. FIG. 3 (b) is a view showing theresults of Co (cobalt) element mapping (apparatus JSM-7800F manufacturedby JEOL Ltd.) over the active material, and FIG. 3 (c) is a view showingthe results of Nb (niobium) element mapping (the same apparatus). Fromboth figures, it was confirmed that niobium was surely adhered (coated)to the active material which was lithium cobalt oxide.

Further, the battery characteristics were evaluated in the followingmanner.

As a positive electrode material, 2.67 ml of N-methyl-2-pyrrolidone(NMP) was mixed in 1.88 g of the sample powder (positive electrodeactive material) and 0.12 g of acetylene black (manufactured by DenkiKagaku Kogyo Co., Ltd.), and the mixture was stirred for 5 minutes usinga homogenizer. Next, 0.33 ml of a 12 wt % PVDF/NMP solution (#1100)(manufactured by Kishida Chemical Co., Ltd.) was mixed and the mixturewas stirred for 5 minutes using the homogenizer, to thereby obtain apositive electrode slurry. The positive electrode slurry was applied onan aluminum foil, using an applicator having a slit width of 200microns. This aluminum foil was dried at 90° C. for 1 hour using a hotplate and further dried at 120° C. for 6 hours in a vacuum dryer. Theobtained positive electrode was pressed with a pressure molding machineand used. At this time, a thickness of the positive electrode layer was50 μm.

Metal lithium was used as a negative electrode, an electrolytic solutionobtained by dissolving 1 mol/L of lithium hexafluorophosphate (LiPF₆) asan electrolyte in a solvent in which ethylene carbonate (EC) anddimethyl carbonate (DMC) were mixed at a volume ratio of 1:2, was used.

As described above, as the battery characteristics, the value (changerate) obtained by dividing a discharge capacity B at the time ofdischarging at a high rate (3 C) by a discharge capacity A at the timeof discharging at a low rate (0.1 C), was used. The larger this valueis, the more smoothly the exchange of lithium ions of the activematerial is performed, which means that battery resistance is low. Inexample 1, the discharge capacity A was 155 mAh/g, the dischargecapacity B was 110 mAh/g, and the change rate was 71%.

As a result of examining the battery characteristics also in comparativeexample 1, the discharge capacity A was 145 mAh/g, the dischargecapacity B was 80 mAh/g, and the change rate was 55%, and the batterycharacteristics were clearly inferior to those of example 1.

Further, DC resistance was measured as another indicator of theindicator representing the battery characteristics. This is theindicator showing that the lower the DC resistance is, the more smoothlythe exchange of lithium ions of the active material is carried out, andit is possible to measure more precisely whether the batterycharacteristics are excellent.

Specifically, the measurement was performed by the following procedure.

<Procedure 1>

A battery discharged at a high rate (3 C) (that is, the battery used forexamining the change rate in the discharge capacity) was continuouslycharged at a constant current of 0.1 C up to 4200 mV. When the voltagereached 4200 mV, constant voltage charge was performed until a currentvalue reached 0.001 C.

<Procedure 2>

Thereafter, the voltage when maintained for 20 minutes (in an opencircuit state) was defined as “an initial voltage”.

<Procedure 3>

Voltage at the time of discharging for 10 seconds at 1 C current (about2 mA) was measured and this value was defined as “voltage after 10sec.”.

<Procedure 4>

The value of DC resistance was calculated by the following calculationformula.“Initial voltage”−“voltage after 10 sec.”=ΔVΔV/current value=DC resistance

In example 1, the above formula was calculated, and the value of theobtained DC resistance was found to be 32Ω.

Hereinafter, the results of each example and each comparative exampleare summarized in table 1 (described below).

Example 2

A solution was obtained, containing lithium and a peroxo complex ofniobium obtained in example 1, in which ammonia was removed and citricacid was added by adding 0.0059 g (0.01 wt %) of citric acid monohydrate(reducing compound, namely stability improver) while stirring theaqueous solution from which excessive ammonia was removed. The residualamount of ammonia was equivalent to that of example 1.

As a result of examining the battery characteristics in the same manneras in example 1, in example 2, the discharge capacity A was 155 mAh/g,the discharge capacity B was 110 mAh/g, and the change rate was 71%. Thechange rate was equivalent to that of example 1. Further, as a result ofexamining the value of DC resistance in the same manner as in example 1,it was found to be 30Ω.

Further, even after storage at 25° C. for 1 month in this solution aswell, no precipitate was formed due to decomposition of the niobiumcomplex.

Example 3

Example 1 was repeated except that the amount of zeolite was reduced to2 g. An amount of ammonia contained in the obtained solution was 1.0mass %. Even in a case of a long storage of 1 month at 25° C. in thesame manner as in example 1, no precipitate was formed.

As a result of examining the battery characteristics in the same manneras in example 1, in example 3, the discharge capacity A was 150 mAh/g,the discharge capacity B was 110 mAh/g, and the change rate was 67%.Further, as a result of examining the value of the DC resistance in thesame manner as in example 1, it was found to be 79Ω. Although the changerate was slightly lower and the DC resistance was higher than those ofexample 1, sufficient characteristics were shown for practical use.

Example 4

Example 1 was repeated except that the amount of zeolite was increasedto 5 g. An amount of ammonia contained in the obtained solution was0.05%. Even in a case of a long storage of 1 month at 25° C. in the samemanner as in example 1, no precipitate was formed.

As a result of examining the battery characteristics in the same manneras in example 1, in example 4, the discharge capacity A was 155 mAh/g,the discharge capacity B was 115 mAh/g, and the change rate was 74%. Thechange rate was further improved compared with example 1. Further, as aresult of examining the value of the DC resistance in the same manner asin example 1, it was found to be 17Ω, and the DC resistance wasdecreased as compared with example 1.

Example 5

Example 1 was repeated except that the amount of zeolite was furtherincreased to 6 g. An amount of ammonia contained in the obtainedsolution was 0.008%. Even in a case of a long storage of 2 months at 25°C. in the same manner as in example 1, no precipitate was formed, and alongest time of storage was possible.

As a result of examining the battery characteristics in the same manneras in example 1, in example 5, the discharge capacity A was 155 mAh/g,the discharge capacity B was 120 mAh/g, and the change rate was 77%. Thechange rate was equal to or better than that of example 4. Further, as aresult of examining the value of DC resistance in the same manner as inexample 1, it was found to be 15Ω, and the DC resistance was decreasedas compared with example 1.

Comparative Example 2

A hydrogen peroxide aqueous solution was prepared, in which 16.0 g ofhydrogen peroxide water having a concentration of 30 mass % was added to19.7 g of pure water. 0.53 g of metal titanium powder was added to thehydrogen peroxide aqueous solution. After addition of the metal titaniumpowder, the temperature of the liquid to which the metal titanium powderwas added was adjusted so that the liquid temperature was within a rangeof 20° C. to 30° C. To this liquid to which the metal titanium powderwas added, 2.4 g of aqueous ammonia having a concentration of 28 mass %was added and sufficiently stirred, to thereby obtain a transparentsolution.

In a nitrogen gas atmosphere, 0.38 g of lithium hydroxide.monohydrate(LiOH.H₂O) was added to the obtained transparent solution, to therebyobtain a transparent aqueous solution containing lithium and a peroxocomplex of titanium. The molar ratio of atoms in lithium to titaniumatoms in the metal complex contained in this aqueous solution is 1.2.

Thereafter, the aqueous solution containing lithium and a titaniumcomplex was allowed to stand for a predetermined time (6 hours to 168hours) at a temperature of 25° C. and whether or not a precipitate wasformed was visually confirmed. When the precipitate was formed, theliquid was stirred to such a degree that the precipitate was dispersed,and thereafter filtration was performed using a membrane filter having apore size of 0.5 μm, to thereby obtain the solution containing lithiumand a peroxotitanic acid complex. The amount of ammonia at this time was1.5 mass %.

Further, as a result of examining the battery characteristics in thesame manner as in example 1, it was found that in comparative example 2,the discharge capacity A was 140 mAh/g, the discharge capacity B was 70mAh/g, and the change rate was 55%. Further, as a result ofinvestigating the DC resistance value in the same manner as in example1, the value was as high as 115Ω.

Example 6

4 g of zeolite (Itaya zeolite Z-13 manufactured by ZEEKLITE Co.) wasadded to a transparent aqueous solution containing lithium and theperoxo complex of titanium (before being allowed to stand at 25° C. fora predetermined time) in which 0.38 g of lithium hydroxide monohydrate(LiOH.H₂O) of comparative example 2 was mixed, and the mixture wasstirred for 30 minutes and centrifuge filtered, to thereby obtain asolution in which ammonium ions in the solution were removed. At thistime, the amount of ammonia was 0.5 mass % (the water adsorbed onzeolite was supplemented with pure water, and adjusted to the sameweight as the solution before the removal treatment).

Unlike a colloidal solution (sol solution), this solution was atransparent solution in which Tyndall phenomenon due to scattered lightwas not observed. Also, even after this solution was stored at 25° C.for 1 month, no precipitate was formed due to decomposition of thetitanium complex, and the solution was remained as a transparentsolution.

As a result of examining the battery characteristics in the same manneras in example 1, in example 6, the discharge capacity A was 150 mAh/g,the discharge capacity B was 100 mAh/g, and the change rate was 67%. Thechange rate was more improved than that of comparative example 2.Further, as a result of examining the DC resistance value in the samemanner as in example 1, it was found to be 48Ω, and the DC resistancewas decreased as compared with that of comparative example 2.

Example 7

After the ammonium ion was removed in example 9, a sample was preparedin the same manner as in example 1 except that excessive hydrogenperoxide in the solution was removed by irradiating it with ultravioletlight (UV-LED device manufactured by Eye Graphics Co., Ltd., wavelength:365 nm) for 60 minutes. The precipitation in the solution did not occurover 2 months, and therefore it was found that the stability of thesolution was further improved as compared with example 1. As a result ofexamining the battery characteristics in the same manner as in example1, in example 7, the discharge capacity A was 155 mAh/g, the dischargecapacity B was 110 mAh/g, and the change rate was 71%. It can be saidthat the change rate was further improved as compared with example 1.Further, as a result of examining the DC resistance value in the samemanner as in example 1, it was found to be 20Ω, and the DC resistancewas reduced as compared with that of example 1.

Example 8

After the ammonium ion was removed in example 2, a sample was preparedin the same manner as in example 2 except that excessive hydrogenperoxide in the solution was removed by irradiating it with ultravioletlight (UV-LED device manufactured by Eye Graphics Co., Ltd., wavelength:365 nm) for 60 minutes. The precipitate was not formed in the solutionover 2 months. Therefore, it was found that the stability of thesolution was further improved as compared with example 1. As a result ofexamining the battery characteristics in the same manner as in example1, in example 8, the discharge capacity A was 155 mAh/g, the dischargecapacity B was 110 mAh/g, and the change rate was 71%. It can be saidthat the change rate was further improved as compared with that ofexample 1. Further, as a result of examining the DC resistance value inthe same manner as in example 1, it was found to be 22Ω, and the DCresistance was reduced as compared with that of example 1.

Example 9

After the ammonium ion was removed in example 4, a sample was preparedin the same manner as in example 1 except that excessive hydrogenperoxide in the solution was removed by irradiating it with ultravioletlight (UV-LED device manufactured by Eye Graphics Co., Ltd., wavelength:365 nm) for 20 minutes. The precipitate was not formed in the solutionover 2 months. Therefore, it was found that the stability of thesolution was further improved as compared with example 1. As a result ofexamining the battery characteristics in the same manner as in example1, in example 9, the discharge capacity A was 155 mAh/g, the dischargecapacity B was 110 mAh/g, and the change rate was 71%. It can be saidthat the change rate was further improved as compared with that ofexample 1. Further, as a result of examining the DC resistance value inthe same manner as in example 1, it was found to be 13Ω, and the DCresistance was reduced as compared with that of example 1.

Example 10

After the ammonium ion was removed in example 9, a sample was preparedin the same manner as in example 1 except that excessive hydrogenperoxide in the solution was removed by irradiating it with ultravioletlight (UV-LED device manufactured by Eye Graphics Co., Ltd., wavelength:365 nm) for 60 minutes. The precipitate was not formed in the solutionover 2 months. Therefore, it was found that the stability of thesolution was further improved as compared with example 1. As a result ofexamining the battery characteristics in the same manner as in example1, in example 10, the discharge capacity A was 155 mAh/g, the dischargecapacity B was 110 mAh/g, and the change rate was 71%. It can be saidthat the change rate was further improved as compared with that ofexample 1. Further, as a result of examining the DC resistance value inthe same manner as in example 1, it was found to be 16Ω, and the DCresistance was reduced as compared with that of example 1.

Example 11

Example 1 was repeated except that the amount of zeolite was increasedto 4.5 g. The amount of ammonia contained in the obtained solution was0.2 mass %. Even in a case of a long storage of 1 month at 25° C. as inthe same manner as in example 1, no precipitate was formed. As a resultof examining the battery characteristics in the same manner as inexample 1, in example 11, the discharge capacity A was 150 mAh/g, thedischarge capacity B was 110 mAh/g, and the change rate was 71%. Thechange rate was equivalent to that of example 1. Further, as a result ofexamining the DC resistance value in the same manner as in example 1, itwas 20Ω, and the DC resistance was reduced by about 40% as compared with32Ω of example 1.

Example 12

Example 1 was repeated except that the amount of zeolite was increasedto 4.8 g. The amount of ammonia contained in the obtained solution was0.1 mass %. Even in a case of a long storage of 1 month at 25° C. in thesame manner as in example 1, no precipitate was formed. As a result ofexamining the battery characteristics in the same manner as in example1, in example 12, the discharge capacity A was 150 mAh/g, the dischargecapacity B was 115 mAh/g, and the change rate was 74%. The change ratewas equivalent to that of example 1. Further, as a result of examiningthe DC resistance value in the same manner as in example 1, it was foundto be 18Ω, and the DC resistance was reduced by about 40% as comparedwith 32Ω of example 1.

Table 1 below summarizes the test conditions before addition of zeolite(and before ultraviolet irradiation) in each example and eachcomparative example, and Table 2 below summarizes the test conditions ofaddition of zeolite (at the time of ultraviolet irradiation) and theresults thereof.

TABLE 1 Hydrogen peroxide Addition amount Pure water solution (30%)Niobic acid Ti powder Ammonia water Lithium hydroxide of citric acid (g)(g) (g) (g) (g) (g) (g) Example 1 33.5 20 2.01 — 3.3 0.46 — Example 233.5 20 2.01 — 3.3 0.46 0.0059 Example 3 33.5 20 2.01 — 3.3 0.46 —Example 4 33.5 20 2.01 — 3.3 0.46 — Example 5 33.5 20 2.01 — 3.3 0.46 —Example 6 19.7 16 — 0.53 2.4 0.38 — Example 7 33.5 20 2.01 — 3.3 0.46 —Example 8 33.5 20 2.01 — 3.3 0.46 0.0059 Example 9 33.5 20 2.01 — 3.30.46 — Example 10 33.5 20 2.01 — 3.3 0.46 — Example 11 33.5 20 2.01 —3.3 0.46 — Example 12 33.5 20 2.01 — 3.3 0.46 — Comparative 33.5 20 2.01— 3.3 0.46 — example 1 Comparative 19.7 16 — 0.53 2.4 0.38 — example 2

TABLE 2 Zeolite Ultraviolet Residual Residual amount irradiation NH₃H₂O₂ (g) (minute) (%) (% or ppm) Precipitate Foaming Example 1 4 0 0.3%3% Absent over 1 month Present Example 1 4 0 0.3% 3% Absent over 1 monthPresent Example 1 2 0   1% 3% Absent over 1 month Present Example 1 5 00.05%  3% Absent over 1 month Present Example 1 6 0 0.008%  3% Absentover 2 month Present Example 1 4 0 0.5% 3% Absent over 1 month PresentExample 1 4 60 0.2% 20 ppm Absent over 2 month Absent Example 1 4 600.2% 20 ppm Absent over 2 month Absent Example 1 5 20 0.05%  0.1% (1000ppm) Absent over 2 month Absent Example 1 5 60 0.02%  20 ppm Absent over2 month Absent Example 1 4.5 0 0.2% 3% Absent over 1 month PresentExample 1 4.8 0 0.1% 3% Absent over 1 month Present Comparative AbsentAbsent 1.5% 3% — Present example 1 Comparative Absent Absent 1.5% 3% —Present example 2 DC resistance Voltage Discharge Discharge Change aftercapacity A capacity B rate Initial voltage 10 sec ΔV Current Resistance(mAh/g) (mAh/g) (%) (mV) (mV) (mV) (mA) (Ω) Example 1 155 110 71 41924128 64 2.0 32 Example 1 155 110 71 4191 4131 60 2.0 30 Example 1 150100 67 4186 4020 166 2.1 79 Example 1 155 115 74 4193 4156 37 2.2 17Example 1 155 120 77 4193 4165 28 1.9 15 Example 1 150 100 67 4190 409595 2.0 48 Example 1 155 110 71 4186 4143 43 2.1 20 Example 1 155 110 714184 4141 43 2.0 22 Example 1 155 110 71 4189 4164 25 2.0 13 Example 1155 115 74 4192 4161 31 2.0 16 Example 1 155 110 71 4188 4145 43 2.1 20Example 1 155 115 74 4190 4155 35 2.0 18 Comparative 145 80 55 4185 3998187 2.0 94 example 1 Comparative 140 70 50 4192 3951 241 2.1 115 example2

Particularly when comparing examples 1, 11 and 12, it becomes clear thatwhen the amount of the residual ammonia is 0.2 mass % or less, the DCresistance value is dramatically decreased.

As a result thereof, according to the abovementioned each example, itwas possible to obtain a solution excellent in battery characteristicsand excellent in handling property when the solution was stored. As aresult, when this solution was used to surface-treat (coat) the activematerial for a secondary battery, it is expected that the coating amountcan be easily controlled.

The invention claimed is:
 1. A solution containing lithium, a metal complex comprising at least one of a niobium complex and a titanium complex, and ammonia, wherein an amount of the ammonia in the solution is 0.2 mass % or less and 1 ppm or more.
 2. The solution according to claim 1, wherein the metal complex has a peroxy group.
 3. The solution according to claim 2, wherein a molar ratio of atoms in the lithium to atoms of a metal in the metal complex is 0.8 to 2.0.
 4. The solution according to claim 1, which further contains a reducing compound.
 5. The solution according to claim 4, wherein a ratio of the reducing compound in the solution is 0.01 mass % to 5.0 mass %.
 6. The solution according to claim 1, wherein the amount of the ammonia in the solution is 0.1 mass % or less and 1 ppm or more.
 7. A method for producing an active material for a secondary battery, comprising: performing a surface treatment using the solution of claim 1; and heat-treating the surface-treated active material.
 8. The method according to claim 7, wherein the active material is an oxide containing lithium.
 9. The method according to claim 7, wherein the active material is an oxide containing lithium, and at least one of a lithium niobate compound and a lithium titanate compound is attached to a main surface of the active material.
 10. A method for producing a solution, comprising: forming a metal complex in a solution by mixing at least one of niobic acid and titanic acid with ammonia; mixing the metal complex and a lithium compound in the solution; and removing the ammonia in the mixed solution until an amount of the ammonia is reduced to 0.2 mass % or less and 1 ppm or more. 